Losing compression. Do I need to bore my cylinders?

Discussion in 'Engine, Fuel and Exhaust' started by sincityharley, Feb 1, 2011.

  1. sincityharley

    sincityharley Member

    Im getting ready to do a top end rebuild on an 02 FLSTCI to replace some of the leaking gaskets. My bike suffers from compression loss. I was reading that its not a bad idea to replace the piston rings. As I was reading this page:
    Compression and How To Check It - Harley Davidson Community

    I came across this quote:

    Bad compression isn't the end of the world, and your motorcycle may run for many miles as is, but for the best power and mileage it's best to bore the cylinders, replace the pistons and rings, and do a valve job.

    I was just wondering. Is it really necessary to bore the cylinders?? Also cant the same pistons be reused?
  2. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

    If you have done a compression test and the results are poor put a few drops of oil in the plug holes and re test if the compression is higher then cylinder and ring wear is part of the problem if it does not change then the valves are the area that need attention
    when doing a compression test always have the throttle wide open to get a good reading
    if you do need a re bore then new pistons and rings will be required to match the new bore size

  3. Knotwillig

    Knotwillig Member

    What is the mileage of your scoot. If your mileage is below 100K Mile it could be another issue like leaking gaskets.

    I would carefully check before going into the hardware.
  4. ultra...good

    ultra...good Banned

    You need to measure the bore size when it is opened up and check what the outer limits are for the pistons in that bore. There are specifications as to when it is recomended that you rebore the cylinders and put new pistons in, compared to just have them honed and reinstalling old pistons with new rings. When I opened up my XL I was still in an acceptable range to re-install the old pistons. However, I could afford to replace them and would rather replace them while I had the motor open to just replace the gaskets, rather then have to take it all apart again two years from now. Also, I only ride the XL when I want to feel some lateral G's so I prefer to keep my compression in the cylinder, not in the cases. There are a few different factors but the best place to start is to see if the bore is still in specification, and if so, by how much? Then decide what you can afford and or are willing to spend. As always, just my opinion.
  5. Jack Klarich

    Jack Klarich Expert Member

    If you measure little taper in the cylinders you may get by with re ringing it, be sure to measure the pistons as well. A valve job would be a big plus as long as the heads are off JMO
  6. TQuentin1

    TQuentin1 Well-Known Member Staff Member Moderator

    Measure the jugs to see what condition they are in. As long as they are still useable with no scoring, then have them honed and cross-hatched. New rings (over-sized if required) and valve job should be enough. If however, your jugs come back shot or scored, then decide if boring or replacing is the better option. At that point you are faced with the up-grade decision for more cubes. If doing that, then you will be eliminating current jugs, heads and pistons, so don't spend any money on stuff until you know what your path forward is.


    BUBBIE Well-Known Member

    It is kinda like this for me, I know MY answer.. NO

    Would You take a good hot shower and then put the same undies and socks back on afterward? :bigsmiley5:

    IF SO you have YOUR answer

  8. geezer

    geezer Senior Member Contributor

    Good explanation Bubbie. Even I could understand that one.
  9. FishHunter

    FishHunter Active Member

    Before any wrenching begins, perform a leakdown test to determine if it is rings, intake valve(s), or exhaust valve(s).:s
  10. Mad Dog Jim

    Mad Dog Jim Banned

    If it were me I'd do a total top end. Seems like the best time to do it. I "almost" envy you.